The present invention relates to an electromagnetic flowmeter having pipe connecting flanges at both ends of its measuring pipe, for electrically measuring a flow rate of a fluid which flows through a pipe and, more particularly, to an improvement of a case structure welded and fixed to the ring-like collar portions on the inner surface sides of both flanges so as to cover excitation coils around the measuring pipe.
A conventional electromagnetic flowmeter utilizes Faraday's law of electromagnetic induction and converts a flow rate of a conductive fluid passing through a measuring pipe into an electrical signal. Various flowmeters have been known as electromagnetic flowmeters of this type. Generally, a measuring pipe of this type comprises a nonmagnetic metal measuring pipe which has pipe connecting flanges at both ends so as to be inserted in a portion midway along a pipe through which a fluid flows. There is an insulating lining which is made of an insulating material such as Teflon or rubber and is formed to cover the inner surface of the measuring pipe. A pair of excitation coils are wound substantially in the form of a saddle and arranged to vertically sandwich the outer surface of the measuring pipe thereby generating a magnetic field in a direction perpendicular to the direction of the fluid flowing through the measuring pipe. A pair of electrodes are inserted and fixed in electrode mounting portions having electrode insertion holes respectively formed in portions of the wall of the measuring pipe so as to oppose each other. There are liquid contact ends opposing each other in a direction perpendicular to the directions of the fluid and the magnetic field generated by the excitation coils. A cylindrical case is arranged around the measuring pipe so as to cover the excitation coils. As is known, such a measuring pipe is inserted in a portion midway along a pipe through which a fluid flows by respectively fastening and fixing the above-described flanges to pipe side flanges by using bolts, nuts, and the like. When the conductive fluid to be measured flows through a magnetic field generated by the excitation coils, an electromotive force is generated in the fluid. The electromotive force is then extracted from the electrodes to measure the flow rate of the fluid flowing through the measuring pipe.
Especially in the above-described electromagnetic flowmeter, a fully welded structure obtained in a simplifying arrangement has been proposed. According to this structure, a case designed to surround a measuring pipe is welded and fixed to the ring-like collar portions or the like extending from the inner surfaces of the pipe connecting flanges. With this structure the case can be made of an iron plate so that the cost can be reduced and the strength is increased. If pipe connecting flanges are selected in accordance with a pipe, since other components and the like can be standardized, this structure is advantageous in terms of mass production. In addition, this structure is not easily influenced by bolts, nuts, and the like for connecting flanges to a pipe, and a space for storing excitation coils, their mounting bands, and the like to be attached to the outer surface of a measuring pipe can be ensured to a certain degree.
In the electromagnetic flowmeter having the above-described fully welded structure, excessive stress concentration is locally caused in the case and the ring-like collar portions or the like for mounting the case due to an external force during a pipe connecting period for connecting the flanges at both ends of the measuring pipe to fluid pipe flanges, thus posing a problem in terms of strength. Even damage and the like of these components may occur. Stress concentration during this pipe connecting period is conspicuous at portions of the ring-like collar portions extending from the flanges and serving as case bonding portions, and hence large bend occurs at the bonding portions between the collar portions and the case. Therefore, demand has arisen for a countermeasure to reduce this stress concentration.
In order to ensure the strength at the bonding portions between the collar portions and the case, the flange, the ring-like collar portions, and the like of the measuring pipe are conventionally formed to have thicknesses with enough margins. In such a conventional structure, however, stress concentration at the bonding portions between the collar portions and the case cannot be prevented. In order to prevent this, each component must be further increased in thickness and size. Such an increase in thickness poses problems, e.g., an increase in overall weight and cost.
Furthermore, in order to prevent heat influences upon welding/fixing of the ring-like collar portions to the case from being transmitted to an insulating lining, the collar portions are desired to be formed to have a thickness as small as possible. In consideration of strength, the thickness of the case is conventionally increased to reinforce the structure, and hence the above-described problems become conspicuous. Therefore, demand has arisen for a countermeasure to reduce the above stress concentration in consideration of these points.